This invention relates to coating compositions based on water-borne reaction product of a carboxyl-functional polymer, an epoxide, and a tertiary amine having general utility in electrocoating metallic substrates. It is more particularly directed to coating compositions useful as can coatings for interior and exterior surfaces of cans and for metal sheet stock.
It is known (U.S. Pat. No. 3,962,060--Brasko et al., 1976 and U.S. Pat. No. 3,933,611--Dudley, 1976) that electrocoating is an excellent method of applying a water-based coating to a continuous coil of metal sheet stock. With conventional coatings, the application technique limit the speed of the coating line generally to a maximum 500-600 ft/min. However, with electrocoating as a means of application, it has been demonstrated that this is not the case. Line speeds above 1000 ft/min are feasible. Other advantages of electrodeposition over other methods such as roller coating for continuous coil application are (1) elimination of the need for coil flattening and straightening before coating to obtain uniform coating build and continuity; (2) pretreatment and pretreatment ovens are not needed; (3) more uniform film coverage so that lower film weights are possible; (4) less solvent emission from ovens and less heat needed for ovens because of low amount of solvent and water in unbaked film; and (5) faster curing times because amine and water are removed during electrodeposition.
It is also known (U.S. Pat. No. 4,094,760--Smith et al., 1978) that electrodeposition is an excellent method of applying water-based coatings to the interior and exterior surfaces of metal containers. There are many advantages to using electrocoating, with the most important being that much lower coating weights can be used.
The electrocoating of metal stock or cans discussed above requires very special resin compositions. The short coating times, in the order of milliseconds to seconds, requires resin systems with high equivalent weights for high coulomb yields so that proper coating weights can be obtained in these short coating times without very high current densities and corresponding electrical power requirements. (A. H. Bushey, Journal of Coating Tech, Vol. 48, No. 619, August, 1976).
Some other requirements of the resin system are (1) short curing times, (2) the proper hardness of the electrocoating film before baking since the unbaked coating in some cases is in contact with rollers or is wiped with a squeegee, and (3) the cured coatings must meet several requirements such as not imparting taste to or subtracting taste from the contents of containers, adequate ease of formability and the ability to go through pastuerization or sterilization conditions.
There are very few conventional electrocoating resin systems that fulfill all of the requirements stated above. The conventional resins are usually low in molecular weight or low in Tg (glass transition temperature) to provide adequate flow requirements during electrocoating and cure. These resins usually require long baking times to obtain proper crosslinked films and also because of their low molecular weights they have low coulomb yields and are too soft in the unbaked state.